1. Field of the Invention
The present invention relates to a prism unit that is used for a reflection-type projector apparatus or the like.
2. Description of the Related Art
Conventional reflective liquid crystal projectors which implement color separation and color composition with an optical system using a plurality of reflective liquid crystal panels for chromatic light generally comprise optical systems of the color separation or the color composition by using a polarized light beam splitter prism or dichroic prism. These optical systems use a plurality of prisms, and a relative positional relationship among the light incident prism surfaces directly influences the image quality, such as contrast deteriorations due to the prism's falls and positional shifts of pixels of each chromatic light, or a shift of the illumination optical axis to the liquid crystal panel.
Accordingly, a prism base used to constitute an integral prism unit by attaching the prisms needs to have a structure which can restrain relative positional changes of the prisms as much as possible. Usually, prisms are bonded and fixed onto the prism base by a UV-curing (ultra violate curing) adhesive, and then attached to a body (see Japanese Patent Laid-Open Nos. 2000-131583 and 2005-208318).
A glass, a metal or a resin may be used for the prism base that joins the prisms in manufacturing a prism unit which can precisely maintain the relative positional relationship among the prisms and have high reliability. Whatever is selected among the glass, metal and resin, if the prisms are bonded to the prism base made of a material different from that of the prism, an elastic deformation due to a difference in coefficient of linear expansion needs to be considered, and the adhesive which bonds the different materials absorbs the relative movements (see Japanese Patent Laid-Open No. 2000-131583).
If an ambient temperature changes due to the bonding condition of the prisms to the prism base, the orientation of the prism may change or the prism may break; when the orientation of the prism changes and the relative positional relationship changes, the optical performance may deteriorate. The ray's reflecting angle may change twice when a polarization beam splitter rotates in the plane perpendicular to the polarization separating surface. The deterioration of the optical performance may also occur when the prism falls in the seating surface direction because the optical axis falls concurrently.